The Red Effect, in People and Monkeys

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The Red Effect, in People and Monkeys

On your next date, wear something red. Or just stand in front of a red background. Research shows that people find members of the opposite sex more attractive when they are in close proximity to the color red. For instance, redder faces are perceived as more attractive and individuals wearing red clothing are considered more sexually desirable. This effect extends to extraneous instances of the color red; even black and white photos of people in red frames are rated as more attractive.

Benjamin Hayden of the University of Rochester and his colleagues wondered if this red effect reflects cultural influences or if there is a more ancient biological basis to it. In many human cultures, the color red is linked to sex and romance. But if the effect is found in other primates, it could reflect a biologically innate sensory bias.

In a new study, Hayden and his colleagues recorded how long rhesus macaques looked at photos of the scrota or hindquarters of opposite sex monkeys on either red or blue backgrounds. Longer gazes indicate more interest in a subject. The researchers also presented the monkeys with a photo of a seashell as a control image.

Both male and female macaques use red body signals in sexual displays. During the mating season, females have redder hindquarters, males have redder scrota, and both sexes have redder faces.

The researchers found that female monkeys looked longer at photos of male scrota on red backgrounds. The effect was not solely due to the red background, as they did not look longer at seashells on a red background. Nor was it specific to scrota, as the females did not look longer at scrota on a blue background.

Neither males nor females showed a preference for the female hindquarters, even when they were framed in red. This finding was surprising, as previous experiments have shown that males look longer at artificially reddened female hindquarters.

Hayden says it's hard to tell exactly what the lack of results with the males means. "Throughout the order of primates, it's very often the males who have the interesting coloration (think of mandrills) and females who attend more to their mates' coloration," says Hayden. "So it may be reflective of a true sex difference - although that's speculation."

It's also possible that males are more interested in the facial redness of females than the redness of their hindquarters, and the photos were too limited to elicit a response.

The results with the female monkeys, at least, suggest that the importance of the color red is ancient. Social cues based on color may be a universal among primates.

"Humans and rhesus macaques diverged 25 million years ago," says Hayden. " The data suggest that we both share a very basic and stable bias towards red in a mating context that goes back at least this far."

Hayden notes that this is a preliminary study, and it needs replicating on a larger scale to see if there really is a sex difference. He also suggests testing other primates — ones that diverged even farther back than macaques and humans, such as lemurs — to see how far back in evolutionary time these sensory biases go.